CYCADALES 



89 



until another cone is produced, when a new meristem appears at the 

 base of the peduncle of the second cone, and the process is repeated 

 every time a cone is produced (fig. 82). A median longitudinal 

 section of the upper part of a large trunk of Dioon spinulosum 

 shows very clearly these domes, each one of which was, in its turn, 

 the apex of the plant (fig. 83). This structure, as presented dia- 

 grammatically by Dr. F. Grace Smith, s^s 

 bears a striking resemblance to Williamsonia 

 (Anomozamites) angiistifolia (fig. 84). 



Naturally, there are no cone domes in the 

 female plant of Cycas, and in those species 

 of Macrozamia and Encephalartos which have 

 axillary cones. There should be cone domes 

 in the male plant of Cycas. What the con- 

 dition may be in Encephalartos and Macro- 

 zamia, when they bear a single apparently 

 terminal cone, remains to be seen. 



The leaf. — The beautiful crown of graceful 

 leaves makes the cycad look like a palm of the 

 Phoenix type, for the leaves are pinnate in all 

 except Bowenia, in which they are bipinnate. 

 In most cycads the leaves come in crowns 

 (fig. 85). The leaves are formed in spiral suc- 

 cession, but it is only when the leaves are 

 very young that there is any noticeable differ- 

 ence in size. In a young crown of a dozen 

 leaves, while the oldest leaf is 30 cm. long, 

 the youngest may measure only 4 or 5 cm. 



The length of the leaf varies from about 3 meters in Cycas circinalis 

 down to 5 or 6 cm. in Zamia pygmaea. Dioon spinulosum has beau- 

 tiful leaves, often 2 meters in length, and in many cycads the leaves 

 are a meter long. The number of leaflets on each side of the rachis 

 varies from more than a hundred in species with large, long leaves 

 down to 3 or 4 in the smallest leaves. In seedlings, even of those 

 species which have a hundred leaflets when mature, there may be 

 only one or two pairs of leaflets. The number then increases gradual- 

 ly with the age and size of the plant. 



Fig. 80. — Dioon spinu- 

 losum: semidiagrammatic 

 view of vascular system 

 of top of stem of a seed- 

 ling, showing the girdling 

 of leaf traces of the first 

 leaf, and also how the 

 leaf trace is built up by- 

 traces from the leaf gaps: 

 /', and l^, ventral strands 

 apparently united with 

 the two dorsal strands. — 

 After Dr. La Dema M. 

 Langdon.3« 



